1. Field of the Invention
The present invention relates to a method for growing GaAs and AlGaAs layers by the use of a molecular beam epitaxial (MBE) apparatus, and more particularly to a method for producing an AlGaAs type semiconductor laser by the use of the MBE apparatus.
2. Description of the Prior Art
In recent years, a method for producing high electron mobility transistors (HEMTs) or semiconductor lasers as an AlGaAs type semiconductor device by the use of an MBE apparatus has been proposed. These devices are produced by epitaxially growing an AlGaAs layer and a GaAs layer on a GaAs substrate.
In the epitaxial growth used for producing an AlGaAs type semiconductor device, the substrate surface is required to be clean and flat in order to grow an epitaxial layer of good quality thereon. The surface of a GaAs substrate may be cleaned, for example, by etching the surface with H.sub.x SO.sub.4 and then evaporating the oxide layer formed on the surface. This evaporating process is performed in the MBE apparatus using an arsenic (As) molecular beam at a high temperature under ultra-high vacuum. An epitaxial layer having a flat surface may be obtained then, for example, by growing a GaAs buffer layer with a thickness of several .mu.m on the substrate surface cleaned as above at a relatively low temperature of 500.degree. to 550.degree. C.
However, the above processes are not applicable to a semiconductor device in which an Al.sub.x Ga.sub.1-x As (0.ltoreq.x.ltoreq.1) layer is required to be grown directly on an AlGaAs substrate without a GaAs buffer layer interposed therebetween.
Moreover, a GaAs buffer layer obtained by the above processes may have poor crystallinity because of defects or impurities along the interface between the substrate and the buffer layer, resulting in unsatisfactory characteristics of the semiconductor device. For example, in the HEMT shown in FIG. 4, poor crystallinity of a GaAs buffer layer 40 reduces the mobility of channel electrons in the active region (a channel region) 41 disposed close to the buffer layer 40.
FIGS. 5(a) to 5(d) illustrate a typical method for producing a semiconductor laser by the use of an MBE apparatus, which is disclosed in Japanese Patent Publication No. 1-37873. In this method, a first growth layer is formed on a semiconductor substrate by the use of an MBE apparatus. The first growth layer includes a first upper cladding layer 123 and a light absorption layer 124. Then, the semiconductor substrate is taken out of the MBE apparatus and a striped groove is formed in a region 128 by photolithography and etching so as to expose the light absorption layer 124. This etching step leaves impurities such as oxides on the exposed surface of the light absorption layer 124. These impurities are then evaporated by irradiation with an As molecular beam at a high temperature in the MBE apparatus. In this evaporating step, the light absorption layer 124 remaining in the region 128 is also evaporated, thereby exposing a very clean surface of the first upper cladding layer 123. Then, a second growth layer including the second upper cladding layer 129 is grown on the first growth layer in the MBE apparatus, resulting in high quality of the second growth layer containing no undesirable impurities.
However, in the above evaporating step using only As as a molecular beam, the surface condition of the exposed light absorption layer 124 greatly affects the surface condition of the resulting first upper cladding layer 123. In addition, the intensity of the As molecular beam, the temperature of the substrate and the shape of the striped groove also affect the evaporating step. Therefore, very limiting conditions are required in order to attain the evaporation satisfactorily. Particularly, when the light absorption layer 124 in the region 128 has unevenly distributed impurities on the exposed surface thereof, the light absorption layer 124 in the region 128 is not evaporated evenly. In other words, the microscopic unevenness generated on the exposed surface of the light absorption layer 124 by the etching step is enlarged by the subsequent evaporating step. In addition, unevenness in the temperature of the substrate or in the intensity of the As molecular beam also results in incomplete evaporation.
Because of these phenomena, the light absorption layer 124 may partially remain along the interface between the first upper cladding layer 123 and the second upper cladding layer 129 in the above-mentioned semiconductor laser structure, thereby adversely affecting the laser oscillation characteristics (i.e., decreasing luminous efficiency) and thus damaging the production yield and/or the uniformity of the semiconductor laser.